Disposable absorbent products like diapers typically include stretchable materials, such as elastic strands, in the waist region and the cuff regions to provide a snug fit and a good seal of the article. Pant-type absorbent articles further include stretchable materials in the side portions for easy application and removal of the article and for sustained fit of the article. Stretchable materials have also been used in the ear portions for adjustable fit of the article. The stretchable materials utilized in these diaper regions may consist of elastomeric films, nonwovens, strands, scrim, etc. Typically, these stretch regions are made separately and attached to the diaper using adhesives. In most cases, these designs deliver uniform and unidirectional stretch, most often in the lateral direction of the diaper.
An alternate approach that is capable of delivering multidirectional, non-uniform stretch has been disclosed in copending Serial application Ser. Nos. 10/288,095, 10/288,126 and 10/429,433. This approach involves hot melt printing of one or more thermoplastic elastomers onto a substrate, followed by incremental stretching of the printed substrate that then confers the stretch properties of the elastomer to the substrate in a somewhat magnified form. Suitable printing processes disclosed therein include direct gravure, offset gravure, and flexographic printing. Each of these printing methods allow deposition of any amount of an elastomer in any shape and direction, thus giving a wide variety of design flexibility which ultimately results in improved fit of the overall diaper product.
In the gravure printing process, a hot melt elastomer is delivered to the cells (also referred to as “grooves”) in a gravure roll via a bath, a slot coater, a sprayer or an extruder. The excess elastomer is doctored off from the roll and the elastomer is then transferred from the gravure cells to the substrate via a nip. Gravure printing is generally used for materials having viscosities less than about 5 Pa·s. Typically, from about 40% to about 60% of the elastomer in the cells is transferred to the substrate. It is understood in the art that the rationale for this diminished transfer is the failure in the gravure cells is cohesive, i.e., the elastomer in the gravure cells splits apart.
Without being limited by theory, it is therefore important to understand the mechanism of transfer of an elastomer from an application means to a substrate. During this transfer, three forces are relevant. These forces include: i) the adhesive force between the surface of the application means and the elastomer; ii) the cohesive strength of the elastomer (i.e., the resistance of a single portion of an elastomeric composition to separation into two smaller portions); and iii) the adhesive force between the elastomer and the substrate and/or the strength of the substrate. In order to successfully transfer an elastomer to a substrate either one or both of the cohesive strength of the elastomer or the adhesive force between the elastomer and the surface of the application means must be less than the adhesive force between the elastomer and the substrate and/or the strength of the substrate. Typically, this problem has been solved by the use of heated printing processes where the cohesive strength of the heated elastomer is at a sufficiently low value because the elastomer has been maintained in a liquid or semi-liquid state. Thus, transfer of an elastomeric composition from an application means to a substrate typically is achieved through cohesive failure of the elastomer at the point of transfer from the application means to the substrate and a portion of the elastomer remains on the surface of the application means. The above conditions generally apply during, for example, gravure printing of elastomeric adhesives, where the viscosity is relatively low and the adhesive has strong affinity for the walls of the gravure elements and also the substrate. Importantly, cohesive failure means that there is a residual portion of adhesive on the application means that is not transferred.
On the other hand, elastomeric compositions that have good elasticity generally have a higher viscosity at a given temperature than a typical elastomeric adhesive. For reference, typical thermoplastic elastomers used in diapers have viscosities in excess of 1000 Pa at 175° C. Increased viscosity translates into a higher cohesive force of the elastomer and a need to heat to a higher application temperature to insure cohesive failure. Such a dynamic poses a problem for conventional direct gravure printing of high viscosity materials, since a point is reached when the cohesive strength of the elastomer either exceeds its adhesive strength with the substrate or it exceeds the strength of the substrate. Such conditions, in turn, result in either a failure of the elastomer to bond to the substrate or damage to the substrate. On the other hand, if temperature is increased to lower cohesive strength, the application temperature of the elastomeric composition may exceed the melting point of the substrate with resulting substrate damage or thermal degradation of the elastomer. Thus, there is a need for an application process that is capable of depositing high viscosity elastomeric compositions on substrates, without damaging these substrates.
Applicants have surprisingly found that printing of high viscosity elastomeric materials would be possible if the conditions during printing are such that the failure inside the gravure cells is adhesive, rather than cohesive, i.e. the adhesive force between the roll and the elastomer is less than the cohesive force of the elastomer and also less than the adhesive force between the elastomer and the substrate. This can be accomplished by one or more of the following: i) using a non-adhesive elastomer that better releases from the cells in the gravure roll; ii) improving the release properties of the gravure roll via providing a release agent, a smoother surface like chrome plating on steel, etc.; iii) increasing the elastomer viscosity, i.e. cohesive strength; and iv) maintaining the gravure roll at a cooler temperature versus the elastomer delivery temperature.
For some materials, when the failure is adhesive, the peel force needed to peel the elastomer from the gravure roll is much lower than when the failure is cohesive. See, Gent and Petrich, Adhesion of Viscoelastic Materials to Rigid Substrates, Proc. Roy. Soc. A, vol. 310, pp. 433-448 (1969). Also, when the failure is adhesive (also referred to as interfacial failure by Gent and Petrich), the peel force needed to peel off the elastomer from the gravure roll is almost independent of viscosity. This a significant benefit, since this process would work even for very high viscosity materials.
When the failure during cell transfer is adhesive, almost all the elastomer is removed from the cells. This substantially complete removal of the elastomer has several advantages over and above the main advantage of high-viscosity printing. First, charring, which is a significant issue with unsaturated elastomers remaining in the dead zones inside the gravure cells, is virtually eliminated. Second, the transfer is uniform since the exact amount deposited within the cells is transferred out each time.
In view of the above outlined approaches, Applicants have determined that a viable approach to increasing the viscosity, and hence the cohesive strength, of the elastomer during cell transfer would be by running the gravure roll significantly cooler than the elastomer delivery temperature.